1. Field of the Invention
The present invention relates to a gas sensor including a stacked-type gas sensor including a plurality of stacked ceramic layers.
2. Description of the Related Art
A plate-shaped gas sensor including a plurality of stacked ceramic layers (solid electrolyte layers) and provided with an electrode (a sensing portion) on a leading end side thereof is known. Such a gas sensor generally includes a ceramic layer having an electrode formed on one surface thereof and a lead portion formed on an opposite side surface thereof, where the electrode and the lead portion are electrically interconnected by means of a through hole conductor formed in a through hole (a penetration hole) penetrating through the ceramic layers (see JP-A-61-134655).
The ceramic layers (solid electrolyte layers) are formed of green sheets. As shown in FIG. 11, after a through hole 241h1 is bored in a ceramic layer 241, an unsintered through hole conductor 206 is formed on an inner peripheral surface of the through hole 241h1, and an unsintered peripheral conductive portion 206S is formed at a periphery of the through hole 241h. Similarly, after a through hole 221h1 is bored in a ceramic layer 221 opposing the ceramic layer 241, an unsintered through hole conductor 226 is formed on an inner peripheral surface of the through hole 221h1, and an unsintered peripheral conductive portion 226S is additionally formed at a periphery of the through hole 221h1.
The ceramic layers 221 and 241 are stacked in such a way that the peripheral conductive portions 206S and 226S of the respective ceramic layers 221 and 241 contact one another, and are thereby electrically connected.
The ceramic green sheet and the unsintered metalized layer differ from each other in the amount of sintering shrinkage. For this reason, deformation, such as warpage in the peripheral conductive portions 206S and 226S, may occur during a sintering operation, and a clearance CL may be generated between opposing members, which results in a deterioration of electrical connection reliability. In this regard, a technique is disclosed for providing elongated conductive portions 247, 222 extending from the respective peripheral conductive portions 206S and 226S to surfaces of the respective ceramic layer 221 and 241 and for establishing electrical connection between the elongated conductive portions 247, 222 which are not subject to warpage (which are not subject to the clearance CL between opposing members) (see JP-A-2008-46112).
Since the cost of conductive materials, such as noble metals, has recently increased, providing the elongated conductive portions 247, 222 as described in JP-A-2008-46112 results in an increase in the amount of conductive material that is used. In FIG. 11, the elongated conductive portions 247 and 222 contact each other (are electrically connected). However, although it is enough to provide electrical conduction between the elongated conductive portions 247 and 222 and the respective peripheral conductive portions 206S and 226S, conductive materials are wastefully used in the elongated conductive portions 247 and 222 shown in FIG. 11.
Also, since the elongated conductive portions contain components different from that of the ceramic layers, adhesion between adjacent ceramic layers may decrease as the portion of the elongated conductive portions increases.